Compact and efficient lasers with a power output of 0.1 to 1.0 Watts (W) or greater, and having single-transverse-mode output beam have a wide range of applications. Such applications include optical communications, laser printing, and optical storage. These applications typically require beam propagation over a distance which is large compared to the size of the laser, focussing the output of the laser into a small spot, or coupling the output of the laser into a single-mode fiber. By way of example, Erbium-doped fiber amplifiers (EDFAs) used in optical communications systems require between about 0.1 W and 0.5 W of continuous-wave (CW) optical pump power in a single mode fiber.
Vertical-cavity surface-emitting semiconductor lasers inherently provide desired circular cross-section output beams. Small diameter VCSELs, for example, less than about 10 micrometers (.mu.m) in diameter, operate in a single transverse mode, however, with an output power limited to less than about 10 milliwatts (mW). For larger devices, for example, greater than about 100 .mu.m diameter, output power can be greater than about 100 mW, however, only in multiple transverse modes. Using an external cavity, i.e., a cavity which is provided by one mirror in contact with a semiconductor gain medium and another mirror spaced-apart from the semiconductor mirror and gain medium, a large diameter (about 120 .mu.m) VCSEL has been forced to operate in a single transverse mode, however, at an output power of only about 2.4 mW.
The above comments regarding prior-art semiconductor lasers are devices which are electrically pumped, i.e., in which carriers are injected across electrical semiconductor junctions to recombine in active layers and thereby generate laser-radiation. In U.S. Pat. No. 5,461,637 to Mooradian and Kuznetsov, an OPS VCSEL operable in a single transverse mode is described. The laser includes a quantum-well structure which provides a gain region. The Mooradian and Kuznetsov patent teaches that an OPS VCSEL can be made to operate in a single transverse mode by separating cavity mirrors of the laser by a solid body which has a significant thermal coefficient of refractive index. The quantum-well structure also has a thermal coefficient of refractive index. Any absorbed pump-radiation which does not contribute to the gain process heats the quantum-well structure and the solid body adjacent thereto. This heating forms in effect a thermal lens in the body. The thermal lens forces the laser to operate in a single transverse mode. A significant drawback of devices described in the Mooradian and Kuznetsov patent is that pump-radiation must traverse one of the cavity mirrors, and, in one arrangement, the solid body also, in order to reach the gain structure. This, together with the thermal lensing effect, provides for difficulties in providing optics which efficiently match pump-radiation with the laser mode diameter at the quantum-well. Because of this, such lasers can be expected to have low optical efficiency.
Further, the VCSEL of Mooradian and Kusnetsov has a quantum-well region including spacer layers of AlGaAs. Because of this, it can be expected that the VCSEL would be subject to problems of limited lifetime similar to those which have been identified in edge-emitting diode-lasers using AlGaAs layers. There is clearly a need for an OPS VCSEL which can operate efficiently at a high power in a single transverse mode but which also has a long operating lifetime.